Fibre-optic transmission systems are now being developed for tens of gigabits-per-second (Gbit/s) communication channels, whilst large volumes of 10 Gbit/s systems are being fully deployed into existing networks. Various potential limits are approached as the performance of such transmission systems is pushed further. The phenomenon of polarisation mode dispersion, PMD, is a problem recently attracting a great deal of attention from the telecommunications industry. PMD is a type of distortion that varies from fibre to fibre and is typically of greater magnitude in older fibres. PMD is also a random phenomenon, varying with both time and optical frequency. While service providers are reluctant to invest in new fibre routes, PMD may restrict the deployment of new systems over the older fibre routes of their network. In a small number of fibres, PMD will give rise to distortions so large that a 10 Gbit/s optical transmission system cannot be reliably deployed over the route. The impact of PMD scales linearly with system bit-rate, hence PMD will become a greater problem as the bit-rate of systems are increased. It is for these reasons that PMD solutions have to be found.
PMD is a fundamental characteristic of both optical fibres and optical components. It arises from the consideration that single mode fibre can actually support two weakly guided modes that are orthogonally polarised. In other words, given an ideal fibre, a pulse can be launched into either of these two polarisation modes and propagate through the fibre in that polarisation mode alone. A fiber exhibits slightly different refractive indices along different axes, a physical characteristic known as birefringence. Birefringence arises from a variety of intrinsic and extrinsic features of the fibre manufacture. These features include geometric stress caused by a noncircular core, and stress birefringence caused by unsymmetrical stress of the core. Other sources of birefringence include external manipulation of the fibre. External forces will include squeezing the fibre, bending the fibre and twisting of the fibre.
In a birefringent fibre, the propagation speed will vary with the launch polarisation state into the polarisation modes of the fibre. Consequently, when proportions of the pulse are launched into both polarisation axes they travel at different speeds and hence arrive at different times. The magnitude of the difference in arrival times between the fastest and slowest paths (along the two Principle States of Polarization (PSPs)) through the fibre is known as the differential group delay (DGD).
The receiver of a direct detection optical transmission system does not distinguish between the different polarisation modes, but simply detects the combination of the different polarisation modes. The difference in arrival times of the pulse through the two polarisation modes will degrade the quality of the received data.
In a long length of fibre the birefringence in expected to be weak but vary randomly along its entire length. This leads to random mode coupling along the fibre, a process by which the pulse will continuously couple power between the two polarisation modes of the fibre. The phenomenon of PMD relates to the random variation of the DGD of the fibre. The DGD is expected to vary randomly over time due to random variations of the fibre birefringence as a result of environmental effects, such as temperature. A consequence of this random variation means that the instantaneous DGD of a fibre cannot be predicted. Instead the DGD of a fibre must be described statistically. The fibre DGD also varies over frequency/wavelength.
The DGD is the first-order consideration of PMD. It makes the assumption that the PMD characteristics of a fibre are constant over the bandwidth of the transmitted data signal. Higher-orders of PMD are considered when the PMD characteristics can no longer be considered constant over the bandwidth of a signal. Higher-order PMD relates to the variation of the PMD characteristics of a fibre with frequency.
In order to compensate for first order PMD, it has been proposed to use a delay line which provides differential delay for different polarisation states, in order to reverse the system fiber DGD. A particular class of fibres, known as high birefringence (Hi-Bi) fibres, has been engineered deliberately to have very high, uniform birefringence for this purpose. The fibres have two clearly definable axes with different refractive indices. The propagation speed of a pulse will differ greatly between each axis.
Three categories of techniques are used for PMD compensations. They are all-optical, all electrical, and hybrid.
For all-optical PMD compensation, the restoration of PMD distortion is done optically without any optical-electrical conversion. The signal remains in the optical domain. Normally, all-optical PMD compensators consist of a polarization controller and a fixed birefringent delay element, such as a piece of high birefringence optical fiber. The basic concept is to align the principal states of polarization (PSP) of the fiber with the principal axes of the birefringent delay element to reverse the DGD of the system fiber.
In the all-electrical method, the distorted optical signal is converted to an electrical signal at the receiver. A delay line filter with specific weights is used to partially compensate for the distortion due to PMD.
Hybrid PMD compensation is a technique that uses both optical and electrical methods to restore the distortion due to PMD. For example a polarization controller (PC) and a polarization beam splitter (PBS) can be used to transform the states of polarization, and split the polarization components. At each output of the PBS, a high-speed photo-detector converts the optical signal to electrical signal. An electrical delay line is used to adjust the phase delay between the two electrical signals.
Problems with the known compensation techniques arise from the need to determine principal states of polarization of the system, and also the need to evaluate the PMD to be corrected.
In polarization bit interleaved (PBI) optical communications systems, adjacent pulses in a transmitted signal have orthogonal polarization. PMD then has the most significant effect when these orthogonal polarizations correspond to the PSPs of the transmission fiber. For bit interleaved signals, the all-optical PMD compensator described above has limited efficacy. PMD compensation is also therefore more difficult for PBI systems.
There are also difficulties in measuring the PMD in a system. Methods for measuring PMD can be broadly categorized in two groups: methods that make measurements in the time domain, and methods that make measurements in the frequency (or wavelength) domain.
The modulation-phase-shift method injects high-frequency sinusoidal intensity modulation into the fiber, and then measures the phase delay of the light exiting the fiber. In performing this test the equipment changes the input state of polarization (during the intensity modulation) to find the maximum and minimum delay. At the maximum and minimum delay the input state of polarization is aligned with the fiber's principal axes. The phase difference between the maximum and minimum delay is then used to determine the amount of PMD at that wavelength. To find the PMD at another wavelength the source can be tuned to another frequency and the test repeated. This method thus measures the instantaneous PMD at a particular wavelength. This method is conceptually quite simple, but it does require the experimental determination of the principal axes. In other words, the measurement must be carried out many times with different input states of polarization in order to determine which states of polarization correspond to the principal axes in the fiber.
The modulation-phase-shift method uses high-speed intensity modulation and phase measurements to directly measure the difference in propagation time for the two principal axes of the device under test.
The pulse-delay method is a direct measurement of the difference in propagation time between pulses launched into the two principal axes. Implementing this method involves launching very short pulses of light into the fiber's two principal axes of polarization and then measuring directly the pulse delay between them. This method, like the modulation phase shift method, also requires experimentally finding the principal axes of polarization.
The above techniques are all time-domain measurements. Frequency-domain measurements use either a source with a broad spectrum (like an LED) or a tunable laser. They make measurements over a wide range of wavelengths. Mathematically, measuring PMD over a wide range of wavelengths gives the same average value of PMD as measuring it at a single wavelength, but over a long period of time. Thus, frequency-domain measurements tend to provide the average PMD value.
Thus, in the past, the measurement of PMD in the system has required complicated analytical processes, particularly requiring test signals to be injected into the component. Furthermore, conventional PMD measurement techniques require the orientation of the system PSPs to be experimentally determined.